1. Field of the Invention
The present invention relates generally to a manufacturing method for an electroluminescent thin film, and more particularly, to a method for manufacturing a light emitting layer of an electroluminescent thin film element to be employed for displays of various electronic devices and of measurement instruments, for terminals of computers and of word processors and so on by using the chemical vapor deposition process.
2. Description of the Related Art
As a material for constituting a light emitting layer of an electroluminescent thin film element, it is known that a semiconductor of II-VI group compound such as ZnS, ZnSe, CaS, SrS is employed as a base material and manganese or rare earth ions such as Tb, Sm, Ce and Eu are added thereto as emission center. Among these materials, the only material currently available for practical use is ZnS:Mn wherein ZnS is employed for the base material and Mn is employed as emission center. In order to grow a ZnS:Mn film, the following two kinds of thin film forming techniques are used:
(1) A vacuum deposition process employing the powders of ZnS and Mn or a mixed sintered material thereof as a evaporation source (for example, Japanese Patent Publication 10358/1977); and PA1 (2) A chemical vapor deposition (CVD) type atomic layer epitaxy process in which Zn, S and Mn are alternately supplied onto a substrate each in the vapor state so as to grow a film in units of atomic layers (Japanese Patent Publication 57-35,158/1982). PA1 (3) A high frequency sputtering technique employing a mixed target of ZnS and Mn; PA1 (4) An organic metal chemical vapor deposition process (MOCVD method) in which an organic compound of II group metal and a hydroxide of VI group metal are thermally decomposed on a substrate; PA1 (5) A Multi-source vapor deposition process in which solid materials of Zn, S and Mn subjected to overheating are supplied from respective evaporation sources under vacuum; and PA1 (6) A halogen transport CVD process in which a hydrogen halide is used as a carrier gas of Mn for emission center.
Besides these methods, the following methods are being investigated in order to improve the quality of an electroluminescent thin film and of making an electroluminescent thin film for multi-color displays:
The applicant of the present invention recently proposed a halogen transport CVD process under low pressure (refer to Japanese Patent laid open Publication 289,091/1989) which is excellent for improving the quality of an electroluminescent film, for enlarging its area and for improving its productivity.
In the halogen transport CVD process, a ZnS source and a metallic Mn source are transported by hydrogen gas (or inert gas) and by hydrogen chloride (HCl) gas, respectively, and a film is grown on a substrate as a result of chemical reaction. A difference of the halogen-transport CVD process at a low pressure from the above-mentioned methods (1) through (5) lies in that the ZnS source gas generated in the source zone (in the upstream side or in a high temperature zone) of the reaction tube is transported to the growth zone (in the downstream side or in a low temperature zone) due to the flow of a carrier gas and the Mn source is transported as a gas of a halogen compound separately. Namely, the ZnS source and Mn source generated according to the following chemical reactions are transported separately to a substrate, so as to grow a film on the substrate due to chemical reaction: EQU ZnS.fwdarw.Zn+(1/2)S.sub.2. (1) EQU Mn+2HCl.fwdarw.MnCl.sub.2 +H.sub.2. (2).
Therefore, the source gases of high purity can be supplied to the substrate, and a film having a close and strong bonding can be grown. Thus, a high quality electroluminescent thin film is obtained.
In the halogen-transport CVD process at a low pressure, the kinds and flow rates of the carrier gases are important factors which affect the film quality and the device characteristics. For example, HCl gas is employed for transport of the Mn source, while a single gas such as H.sub.2 gas or inert gas such as He or Ar is used as the carrier gas of the ZnS source.
As to a practical use of an electroluminescent film, it is demanded to produce an electroluminescent film of better electroluminescent properties with better reproducibility characteristics. In order to obtain better electroluminescent characteristics, it is desired for an electroluminescent film to have a larger concentration of emission center and to have better crystallinity or less crystal defects. The halogen transport CVD process is deficient for these factors.
Accordingly, an object of the present invention is to provide a manufacturing method of an electroluminescent thin film wherein emission center can be doped in a thin film electroluminescent element at a high concentration with good reproducibility characteristics and the crystallinity of a thin film electroluminescent element can be increased to a desirable level.